What was found
(CAN) THE AIRCRAFT WAS IN LEVEL FLIGHT WHEN SUDDENLY WITHOUT ANY WARNING, THE CREW DOOR SEPARATED FROM THE AIRCRAFT. THE PILOT & COPILOT BOTH CONFIRMED THAT THE DOOR WAS CLOSED, LATCHED AND LOCKED CORRECTLY . THEY THEN EXTENDED THE GEAR & DROPPED THE FLAPS TO SLOW THE AIRCRAFT DOWN BECAUSE IT WAS VERY COLD OUTSIDE. THE AIRCRAFT HAD NO PASSENGERS ON BOARD, IT WAS RETURNING BACK TO BASE AFTER DELIVERING A LOAD OF FREIGHT. IT IS HARD TO DETERMINED WHAT HAPPENED WITHOUT THE DOOR. THEY WERE AT 4,000 FEET OVER A LAKE WHEN THEY LOST THE DOOR. THERE ARE NO SIGNS OF FAILURE ON THE AIRFRAME EXCEPT WHERE THE DOOR BROKE OFF (FRESH BREAK).